Lesson 7 Notes: Conditionals --The Power of “If …”

• Conditionals
  • Test if a “condition” is true
    • Did the user type in the right word?
    • Is the number larger than 10?
  • The result of the test decides what happens
    • All numbers larger than 100, subtract 20
    • When correct word is entered, print “yeah!”
• Conditional Tests
  • x= = y
    x is equal to y
  • x != y
    x is not equal to y
  • x > y
    x is greater than y
  • x< y
    x is less than y
  • x >= y
    x is greater than or equal to y
  • x<=y
    x is less than or equal to y
• Examples of Conditional Tests
  • if x == 125:
  • if password == “nix”:
  • if num >= 0:
  • if letter > “L”:
  • if num/2 == (num1-num):
  • if num%5 != 0:
• Analysis of an “If” statement
  if condition:
    Statements
  • if = keyword
  • Condition = a conditional test (x<y, x==y, etc.)
  • : = Required at end of “if” line
  • Statements = instructions to follow if the condition is true. These instructions must be indented.
• Example: Simple “If”
  num = input(“Enter your age”)
if num >= 30:
    old_person(num)
print
print "Thank you for your input"
• If … else
  if condition:
      statements
  else:
      statements
  • “else” must be preceded by an if test.
  • “else” will run only when condition of if statement is not met.
  • Use this construction for two branch with only two choices
• Example: “If … else”
  if x%2 == 0:
    print “The number is even”
else:
    print “The number is odd”
• If … elif … else
  if condition:
    Statements
elif condition:
    Statements
else:
    statements
  • For more than two options, use “elif”
  • “elif” is short for “else if”
  • “else” statement will run if others fail
• Example: “If … elif … else”
  
if x < 0:
    print x, “is negative”
elif x == 0:
    print x, “is zero”.
elif x > 0:
    print x, “is positive”
else:
      print “Error.”, x, “is not a number.”
• Power Keyword: pass
  • Pass
  • A “do nothing” command
if x== 0:
      Do_something()
else:
      pass
• Power Keyword: return
  • Return
  • “Exit the function” command
Def function(num):
    if num < 100:
        print “you must enter a number above 100”.
        return
    do_something()
• Odd Notes
  • “if” tests are especially useful for catching and handling errors. Use "else" to catch errors when no conditions are met.
  • Conditional tests can be nested, but this can get confusing and should be used sparingly.
• Review
  • What is a conditional statement?
  • What is the difference between "else" and "elif"?
  • What command exits a function?
  • What command means "do nothing"?
  • How can "if" test be used to catch errors?
  • What code is used to test for equality? Non-equality (not equal)?